Mold modifications for eliminating freckle defects in roll castings

ABSTRACT

A composite mold for casting a roll having a metal chill at the terminus of the lower journal portion. This improved apparatus is particularly useful in eliminating subsurface defects, known as &#39;&#39;&#39;&#39;freckles&#39;&#39;&#39;&#39; on the machined surface of the body portion of rolling mill rolls.

United States Patent [191 Rohatgi et al.

[451 May 13,1975

1 1 MOLD MODIFICATIONS FOR ELIMINATING FRECKLE DEFECTS IN ROLL CASTINGS [75] Inventors: Pradeep Kumar Rohatgi, Bangalore,

India; Louis R. Woodyatt, Whitehall, Pa.

[73] Assignee: Bethlehem Steel Corporation,

Bethlehem, Pa.

[22] Filed: May 24, 1973 [21] Appl. No.: 363,682

[52] US. Cl. 164/353; 164/355; 164/363;

164/127 [51] Int. Cl B22d 15/00 [58] Field of Search 164/125, 122, 127, 371,

[56] References Cited UNITED STATES PATENTS 891,309 6/1908 West 164/356 West 164/356 Mueller et a1. 249/1 1 l FOREIGN PATENTS OR APPLICATIONS 406,558 11/1924 Germany 164/356 121,548 11/1958 U.S.S.R ..164/356 Primary ExaminerFrancis S. I-Iusar Assistant ExaminerV. K. Rising Attorney, Agent, or Firm-Joseph J. OKeefe; Michael J. Delaney; William B. Rice [5 7 ABSTRACT A composite mold for casting a roll having a metal chill at the terminus of the lower journal portion. This improved apparatus is particularly useful in eliminating subsurface defects, known as freckles on the machined surface of the body portion of rolling mill rolls.

7 Claims, 3 Drawing Figures 1 IIIIIIII'IIII PATENTEDMAY I 31915 Hg. 2 PRIOR ART III VIII

Ilmlll VII 'II 1 MOLD MODIFICATIONS FOR ELIMINATING FRECKLE DEFECTS IN ROLL CASTINGS BACKGROUND OF THE INVENTION This invention relates to vertically casting metal rolls in a composite mold comprising sand and metal chills. It is known that progressive solidification from bottom to top of a casting is very effective in eliminating pipe and other centerline defects closely related to the longitudinal axis of vertically aligned castings. Known means for promoting directional solidification include the use of variable chills, sinkhead insulation and arc heating of sinkheads.

It is also known that chemical reactions between molten metals and molding materials may introduce surface defects in castings which may penetrate the surface of the casting. Known means for avoiding these detrimental chemical reactions are principally associated with the elimination of moisture and the selection of inert refractories.

Defects due to mold reactions are usually visible on the as-cast surfaces or may be encountered in the shallow cuts occurring during initial machining. Deep machining, such as cutting grooves in the body of a roll casting or removing a sinkhead, may reveal defects due to lack of progressive solidification. These latter defects increase in size and number while approaching the longitudinal axis. This invention is directed to solving the problem due to defects which fall between these two areas, i.e. freckles" or relatively shallow subsurface discontinuities which do not extend into the centerline shrinkage. Centerline shrinkage is a shrinkage void located along the longitudinal axis. It occurs during freezing contraction (about 5 percent) and is the result of freezing from two sides in combination with inadequate progressive solidification from bottom to top.

Expired US. Pat. No. 2,104,906 granted to H. Mueller et al forMeans for Casting Rolls is pertinent prior art. This patent is directed to casting apparatus for producing rolls which are free of blow holes, piping, segregations, pockets and other casting faults. The apparatus of this patent positions variable chills and molding sand around the periphery of a roll casting.

SUMMARY OF THE INVENTION The principal object of this invention is to provide an apparatus for casting a mill roll free of sub-surface casting defects.

Applicants have discovered an improved composite mold for vertically casting a roll. This composite mold includes cope, cheek and drag portions, each of which is molded from materials selected to enhance directional solidification of the roll. The cope section is molded in.a refractory sand to provide an insulated hot top to aid in maintaining a pool of molten metal to feed the balance of the casting. The cheek portion comprises a sand backed metal chill positioned to be in direct contact with the body portion of the roll which has the largest diameter of the casting. Positioning the cheek chill in direct contact with the body of the roll serves to accelerate solidification of the body of the roll. The drag portion comprises sand backed chills faced with sand positioned to contact the vertical and horizontal sections of the lower journal portion of the roll casting. The application of an additional chill to the terminus of the horizontal terminus of the lower journal provides a novel unobvious result by eliminating freckle defects in cast rolls.

DESCRIPTION OF THE DRAWINGS PREFERRED EMBODIMENT OF THE INVENTION I The composite mold of this invention as illustrated on FIG. 1 comprises cope l0, cheek 11, and drag 12 sections in three separate flasks. Each flask is an upright cylinder having an exterior flange -13-l8 positioned at each end of the three cylinders. The flanges 15 and 16 of the cheek section are arranged to be fastened to the bottom flange 14 of the cope and top flange 17 of the drag. A metal plate 19 used to close the bottom'of the drag 12 is'also arranged to be fastened to the bottom flange 18 of the drag. A portion of the connections is indicated as being threaded bolts 20 and nuts 21.

The cope 10 section of the mold uses molding sand 22 to form the casting cavity for the hot top 23A and the upper journal 23 portion of the roll casting. The cheek 11 section of the mold comprises a first metal chill 24 backed with molding sand22. This first chill is a hollow cylinder with a straight face forming the inside wall 25 and three steps 31, 32 and 33 forming the outside wall. The thickness of this chill 24 decreases from drag face to the chill face. This first chill 24 may be termed a variable chill, the bore of which forms the casting cavity for the body of the roll casting. The second chill 26 is generally shaped as an open end inverted bell with a flare 34 at the upper end and a straight cylinder 35 projecting from the lower end. The wall of chill 26 may be of constant thickness and forms the vertical section of the lower journal 40 portion of the roll casting. The novel third metal chill 27 is a flat disc forming the horizontal terminus 30 of the lower journal 40 portion of the roll casting. The second chill 26 and the third chill 27 have a sand facing 28 forming the molding cavity for the lower journal 40 end of the roll casting. This composite mold includes a vertical sprue 36 molded in sand and horizontal gate 37 molded of preformed sections 38 bonded with cement for introducing the molten metal into the vertical portion of the lower journal 40 of the casting through a vertical slot in the bottom of the second chill 26.

FIG. 2 illustrates a prior art composite mold comprising a cope 5t), cheek 51 and drag 52 having a vertical sprue 53 and horizontal gate 54. The prior art cope 50 is molded in sand 22', the cheek 51 includes a first metal chill 55 backed with sand 22' and the drag 53 includes a second metal chill 56 having sand 22' for backing and sand 23' for facing the second chill 56.

In practicing this invention applicants have found it desirable to reduce the loss of radiant heat from the sinkhead 23A by arc heating the top surface as indicated by the three electrodes 39 and 56 for FIGS. 1 and 2, respectively.

A comparison of FIGS. 1 and 2 illustrates the novelty of this invention provided by the addition of the third chill 27 in the composite mold of this invention.

FIG. 3 illustrates the contours of rolls machined from roll castings produced with the composite molds of FIGS. 1 and 2. Also indicated on FIG. 3 are the freckle defects 41 common to the rolls produced from composite molds of the prior art as shown in FIG. 2. Rolls produced from composite molds of this invention as shown in FIG. 1 did not have the defect known 'as freckels. Freckles are sub-surface defects generally associated with porosity and microsegregation and appear on machined surfaces as elongated discontinuous spots and streaks in a spaced alignment parallel to the longitudinal axis of the roll casting. Freckles are defects not apparent on as-cast surfaces and may only appear after a significant portion of the machining is completed. Freckles appear closer to the surface than the longitudinal axis of the casting but they are neither continuations of metal-mold reaction defects nor the outermost portions of centerline shrinkage defects.

SPECIFIC EXAMPLE An example of the advantageous use of the mold of this invention is provided by the pair of rolls cast in accordance with FIGS. 1 and 2 of the following steel composition of iron containing alloys by weight per cent as shown below.

Carbon 1 .80% Manganese .55% Phosphorus .0 I Sulfur .00 8% Silicon .37% Nickel l 36% Chromium .97% Vanadium .09%

As indicated by FIG. 1, means for introducing molten metal into the composite mold of this invention is provided by a sprue 36 and ingate 37 connecting with a side of the lower journal portion. In use, the metal is poured through the sprue 36 until the metal in the mold has risen high enough to fill the body of the roll at the joint between the cope l and cheek 11. At this point, pouring is interrupted so as to pour the balance of the mold by pouring the metal through the sinkhead 23A. In the cheek 11, the body portion of the roll, the metal solidifies in contact with the first metal chill 24 and in the drag 12 the metal solidifies in contact with the facing sand 28 of second metal chill 26 and the third metal chill 27.

The prior art composite mold illustrated by FIG. 2 was poured in the same manner, i.e. the molten metal was poured through the sprue 53 until the metal in the mold had risen to the intersection of the cope 50 and the cheek 51. At this point, pouring is interrupted so as to pour the balance of the mold by pouring through the sinkhead.

This pair of roll castings were heat treated and machined in the usual manner to provide a matched pair of grooved rolls having contours illustrated by the sketch of FIG. 3. The cast roll made with the composite mold of this invention was completely free of freckles. Nineteen additional cast rolls made with the composite molds of this invention are in various stages of production and to date all have been completely free of freckles. Detrimental freckle defects are frequently encountered on machined roll castings produced with the prior art composite molds illustrated by FIG. 2.

In practicing this invention Applicants have found it desirable to use a large drag chill 27 having a diameter not less than the diameter of the terminus of the lower journal 30, and preferably a diameter not less than the diameter of the base of the cheek 25 forming the body of the roll casting. In the preferred embodiment Applicants also used granular chromite ore for the facing sand 23 for the second and third chills.

Applicants do not completely understand the exact nature of the manner in which the composite mold of their invention eliminates freckles. It is necessary to select stable refractories to avoid metal-mold reactions which are known sources of surface defects. It is also necessary to promote directional solidification to avoid centerline shrinkage by placing variable chills along the vertical sections of the mold, insulating the sinkhead with sand and adding arc heating to reduce heat loss by radiation. However, the manner in which freckles are eliminated by adding a chill to the terminus of the lower journal is not readily apparent.

We claim:

1. In a mold forming a cavity for vertically casting a rolling mill roll, or the like, having a central body portion separating upper and lower journal portions, a sinkhead positioned above the upper journal portion, sand to contact the sinkhead and upper journal portions, a first metal chill to contact the central body portion, a second metal chill having a facing of sand thereon to contact the vertical section of the lower journal portion and the improvement comprising a third metal chill having a continuous flat upper face and a facing of sand thereon to contact the horizontal terminus of the lower journal portion so as to provide for the casting of a rolling mill roll free of freckle defects.

2. The mold of claim 1 having a sprue and ingate connected to the lower journal portion. 7

3. The mold of claim 1 in which the sand in contact with the metal chill in the lower journal and drag portions comprises granular chromite ore.

4. The mold of claim 1 in which the third metal chill has a diameter of not less than the diameter of the lower journal portion.

5. The mold of claim 1 in which the third metal chill I has a diameter not less than the diameter of the body the facing of sand. 

1. IN A MOLD FORMING A CAVITY FOR VERTICALLY CASTING A ROLLING MILL ROLL, OR THE LIKE, HAVING A CENTRAL BODY PORTION SEPARATING UPPER AND LOWER JOURNAL PORTIONS, A SINKHEAD POSITIONED ABOVE THE UPPER JOURNAL PORTION, SAND TO CONTACT THE SINKHEAD AND UPPER JOURNAL PORTIONS, A FIRST METAL CHILL TOCONTACT THE CENTRAL BODY PORTION A SECOND METAL CHILL HAVING A FACING OF SAND THEREON TO CONTACT THE VERTICAL SECTION OF THE LOWER JOURNAL PORTION AND THE IMPROVEMENT COMPRISING A THIRD METAL CHILL HAVING A CONTINUOUS FLAT UPPER FACE AND A FACING OF SAND THEREON TO CONTACT THE HORIZONTAL TERMINUS OF THE LOWER JOURNAL PORTION SO AS TO PROVIDE FOR THE CASTING OF A ROLLING MILL RO FREE OF FRECKLE DEFECTS.
 2. The mold of claim 1 having a sprue and ingate connected to the lower journal portion.
 3. The mold of claim 1 in which the sand in contact with the metal chill in the lower journal and drag portions comprises granular chromite ore.
 4. The mold of claim 1 in which the third metal chill has a diameter of not less than the diameter of the lower journal portion.
 5. The mold of claim 1 in which the third metal chill has a diameter not less than the diameter of the body portion.
 6. The mold of claim 4 in which the vertical portion of the third chill is equal to the space in the mold below the facing of sand.
 7. The mold of claim 5 in which the vertical portion of the third chill is equal to the space in the mold below the facing of sand. 